Basically there are two types of hair curlers. Hair curlers of a first type are intended to form relatively short-lived curls of moist or dry hair and hair curlers of a second type for producing long-lived curls with the aid of chemical treatments. Both types of hair curlers differ with respect to their diameters. Hair curlers are provided with diameters of 13 mm to 80 mm for the first type of curling.
By contrast, cold waving hair curlers have diameters beginning at 4 mm and have a maximum diameter, with respect to the part of the curler about which the lock of hair is wound, of about 15 mm.
Cold waving is carried out with the aid of chemical treatment of the hair resulting in a more or less permanent or residual structural change therein. The chemical processes involved are complex since they may include a softening of the hair substance with the aid of waving agents and a fixing of the deformed structure with fixing agents. If the wetting of the hair with the waving agent is incomplete, the results are poor.
If the following fixing step is incomplete, the results can be catastrophic, including a total or partial dissolution of the hair substance, the loss of all or part of the hair, etc. The hair curler, therefore, is of considerable importance in hair dressing since it must be complete to ensure full treatment of the hair wound in multiple layers upon the curler, i.e. treatment of those layers which lie closest to the curler body as well as those layers of the lock of hair which may be exposed once the lock is wound on the curler.
The waving and fixing liquids must be capable of penetrating from the upper or outermost layers to the innermost layers and, of course depending upon the length of the hair and the number of turns of the lock on the curler, this has been difficult to ensure with conventional curlers. A further problem with respect to most earlier curlers is that the penetration of the liquid into and through the hair wound on the curler is often impeded by the impenetrability of the body of the curler.
The cost of a cold wave and the time consumed in hair dressing using cold waves are significant and cannot be frequently tolerated by most modern women. There is, therefore, a tendency toward a do-it-yourself approach in which the cold wave is effected at home.
There are numerous products allowing cold waves to be done at home or intended for that purpose. By and large these are relatively complex and expensive.
They include solid plastic curlers which, because of their relatively high weight and the large number of curlers which are required for an effective cold wave, frequently up to 50, create problems in handling.
There are hollow two-part curlers with holes making the curlers significantly lighter but substantially more costly since they require the fabrication of the two halves in the injection molding process separately and the cementing of the two halves together subsequently. In many cases these two-part curlers are also unsatisfactory.